Generally, semiconductor integrated circuits include electrical wires of a conductive material for electrically connecting a plurality of circuit elements which are formed therein. As for the material of the metal wires, aluminum (Al) is widely used. Aluminum has the characteristics of low electrical resistivity, low price and relatively easy processing, and, therefore, aluminum is the most widely used conductive material.
An aluminum alloy containing 10% by weight of Si or Cu may be deposited by a sputtering method, thereby forming a wiring layer for carrying out a metallization process.
In accordance with the trend of high density and high integration of semiconductor devices, however, the step difference of the surface is increased in an underlayer of the wiring layer, and the aspect ratio of the contact holes is increased. Therefore, due to the inherent characteristics of the sputtering method, the angle of sputtered Al atoms which may reach inside of the contact hole is decreased. As a result, the problem of step coverage of a deposited aluminum layer is aggravated, with the result that the wire resistivity is increased, and the electromigration is increased, thereby adversely affecting the reliability of the metallization.
Meanwhile, as methods proposed for improving the deposition characteristics of an Al alloy layer, particularly the step coverage, there are: a method of melting an Al alloy using a laser; a method of carrying out a deposition while heating the wafer; and a method of carrying out a flattening in a dual manner by carrying out a collimator sputtering and a high temperature sputtering for ensuring a straight motion and directionality for the sputtered Al atoms. The above cited methods are for improving the existing sputtering method, and are in various stages of research. Besides attempts to improve the sputtering methods, studies are being conducted to improve the contact hole and step coverage by applying a low pressure chemical vapor deposition method in which the conformability of the deposited layer may be improved.
Particularly, a CVD method may simultaneously satisfy the filling of the contact hole and the conformability of the lines, and selective deposition is possible. Therefore, this method is expected to be a future wire forming method for integrated circuits. An example of an Al CVD method is disclosed in U.S. Pat. No. 5,179,042 (dated Jan. 12, 1993).
If a conductive layer (Al layer) is formed by applying a conventional CVD method, however, the CVD Al layer shows archipelago-like growth characteristics. Further, the CVD method is carried out at a temperature higher than that of the sputtering method. Further, if the wiring becomes fine, the contact areas between the grains are decreased, and, therefore, the electric resistance is increased, as well as causing problems of stress migration or electromigration. Further, within the contact hole, a void is likely to be formed near the center of the contact hole.
Actually, the cross section of the Al layer which is grown on an semiconductor substrate by applying a cold wall type LPCVD apparatus and through a pyrolysis of TIBA (Al(C.sub.4 H.sub.9).sub.3) gas using an Al source can be observed through an SEM (scanning electron microscope). Through such an observation, it can be confirmed that the surface roughness occupies over one half.
If the Al layer which is formed by application of a CVD method is reflowed based on a conventional method, the Al layer varies in accordance with the step of the lower layer. If this layer is patterned into a wiring pattern, line breaks, electromigration or stress migration may occur, thereby lowering the reliability.